It’s possible, crayfish will eat just about whatever they can catch, and mine have always tried to get catfish.
I find it more likely, however, that the pleco is hiding. They can disappear in a tank given enough hiding spots, so I’d give it a bit before thinking him dead.
If she is the culprit (extremely possible and fully predictable), she didn’t swallow it at once. They can’t do that. Having caught it, she shredded it bit by bit as she ate the pieces. If any was left, any and all tank inhabitants helped finishing it up.
I should add that the pleco would have likely been sick/dying when the crayfish got it if it got it. IME crayfish are pretty crappy hunters and can only successfully kill already moribund fish. There are exceptions though.